Bimorph electro optic light modulator

ABSTRACT

There is disclosed herein a bimorph using ultraviolet setting glue to laminate the structure and having etched back metalization patterns on the surfaces of the piezoelectric film used to make the bimorph to minimize the possibility of shorting and to eliminate electrostatic pinning and to increase the mechanical reliability and lifetimes of the electrical contacts. Also disclosed is a method for making the bimorph and a method for registering the bimorph on a light modulator. There is also disclosed a bimorph light modulator consisting of a substrate on which there is attached an fiber optic input light guide and a fiber optic output light guide. There is a gap between these light guides through which light passes as it is coupled between said light guides. A bimorph is affixed to the substrate with a shutter attached to one end of the bimorph such that the shutter is registered in the gap so as to block light coupling when the bimorph is in the unenergized state. When the bimorph is subjected to electrical fields of the proper polarity, the bimorph bends thereby pulling the shutter out of the gap and allowing light coupling. Top and bottom stops are used to limit the bimorph movement to damp resonant vibrations and improve on and off times. Viscous air damping is used to eliminate or minimize bounce of the top and bottom stops and to help damp resonant vibration.

This application is a continuation of application Ser. No. 944,695,filed Dec. 19, 1986, now abandoned.

BACKGROUND OF THE INVENTION

The invention pertains to the field of bimorphs in general, and moreparticularly, to the field of bimorph based light modulators.

Bimorphs are not new devices. Basically, a bimorph is a devicemanufactured with two strips of piezoelectric film which are fastenedtogether and which have electrodes allowing electrical fields of theproper polarity to be applied through the film to cause anelectrostrictive effect to occur. This electrostrictive effect changesthe dimensions of the film of the two different strips in such a waythat the bimorph bends.

Bimorphs have been used in the prior art to modulate light by using thebimorph to bend into and out of the path of a light beam. The physicalocclusion of the light path by the bimorph interrupts the light beam andtherefore modulates the light in accordance with the timing andintensity of the electrical fields applied to the bimorph. Bimorphs havealso been used in the prior art to interrupt the light path at the focalpoint between two lenses, and have been used to trip the shuttermechanisms of cameras when a sufficient amount of light for properexposure has been received. It is known in the prior art to attach aright angle shutter to a bimorph and to allow the bimorph to bend intoand out of the path of light through an aperture in a mask plate. It isalso known to use bimorphs in display elements such as seven segmentdisplays where instead of using light-emitting diodes for each of thesegments in the display, a bimorph painted with a distinctive color isused to activate each display segment. It is also known in the prior artto doubly support the bimorph with a fixed fulcrum somewhere in themiddle of the length of the bimorph and a movable fulcrum on one endthereby leaving one end of the bimorph free to move in response to theapplied electric fields.

It is also known in the prior art to use piezo film for itspiezoelectric property in order to manufacture transducers. That it,when piezo film is subjected to mechanical stress, a voltage can begenerated which can be sensed to signal the occurrence of an eventcausing the mechanical stress.

It is also known in the prior art to use bimorphs in conjunction with aMach-Zehnder optical interferometer to implement an optical phaseshifter in fiber optic sensor systems.

Other workers in the art have used bimorph light beam deflectors whereina mirror is placed at the end of a bimorph and a laser beam is directedonto the mirror such that the angle of the reflection is altered by thebending of the bimorph. Still other workers in the art have usedbimorphs to implement a mechanical multiplexer for fiber optic switchingof light from one input fiber to either of several output fibers.

There are certain problems which arise from the use of bimorphcantilevered beams for the interrupting of light paths. For one, thecantilevered beam has its own mechanical resonant frequency. When such abeam is excited by a narrow pulse or a step function, the beam bends andmechanical resonance or vibration often occurs, causing the free end ofthe beam to vibrate. If the vibration causes any portion of the bimorphor the shutter to move into and out of a light path, errors in thedesired average light flux will occur. Further, when driving such abimorph a high frequencies, the acceleration of the beam and itsvelocity is high. If a mechanical stop is used to limit the travel ofthe beam, the bimorph can hit the mechanical stop and bounce. Suchbouncing action is called chatter and also causes errors in the averagelight flux passing the bimorph since the light flux is calculated underideal conditions where no bounce occurs.

It is useful to use bimorph light modulators in large area displayswhere each bimorph modulates the average light flux emerging from aparticular pixel location or from one color component of a pixel in athree-primary-color pixel. Such displays have advantages overconventional secondary emission displays in that a light source of anydesired intensity may be used to supply the input light which may thenbe modulated using the individual bimorphs in accordance with sceneinformation to control the gray scale light intensity of each pixel. Theadvantage of such an arrangement is that high contrast and goodvisibility during high ambient light conditions can be achieved. Thatis, the light emerging from the face of such a display can be made muchmore intense than the light emerging from secondary emission displayssuch as CRTs and television type screens since phosphor light emissionis limited in intensity. In contrast, a display using bimorphs is notlimited in the intensity of the light at each pixel location by thephysical nature of any secondary emission type material such asphosphor. Further, such a display can be made very large since the lightfrom the source can be directed to very large numbers of pixels byoptical channels such as fiber optic light guides, and there is no needfor deflecting an electron beam to raster scan the entire display.

Use of bimorphs to control the pixel light intensities in a large scaledisplay requires very accurate correspondence between the electronicsignal which encodes the desired amount of light at each pixel locationand the actual average light flux which is gated through to thatlocation by the bimorph. Further, video displays require vast quantitiesof data to be handled in very short times if the display is to becompatible with NTSC and PAL television signals. Thus, each bimorph mustbe able to operate at a fairly high frequency and accurately control theaverage light flux passing through the light channel controlled by thatbimorph.

The bimorph structures taught in the prior art could not be used forapplication to such a large scale display. For one thing, the structurestaught in the prior art suffer from resonance and chatter problems whichwould degrade the accuracy and repeatability in controlling the averagelight flux passing through the light channel controlled by each bimorph.Further, the bimorph structures taught in the prior art would sufferfrom electrostatic pinning problems which would degrade the ability ofthe bimorph to operate at high frequencies necessary to handle NTSC andPAL television signals. The bimorph structures taught in the prior artwould also be unreliable since no means is taught for preventing theelectrostrictive dimensional changes of the bimorph film from occurringat the location of electrical contacts. Thus, the electrical contactscan be rendered intermittent or be caused to fail altogether by themechanical stresses induced when the film changes dimensions under thecontact locations.

Further, the bimorphs of the prior art are generally glued together withglues which render the assembly of the bimorph difficult. It isimportant to be able to glue the two film strips together withoutbubbles, wrinkles or other stress in the film which could cause curl inthe final structure. With the types of glue taught in the prior art,only a limited amount of time is available before the glue sets toadjust the two films and eliminate wrinkles, curls and stresses. Thiswould make assembly and registration of the two films and removal ofwrinkles, bubbles and other stress-producing artifacts more difficult.

Further, the prior art does not teach a method of registering thebimorph and shutter with a light path to insure that complete occlusionof the light path will occur when the bimorph is in the "off" position.Since bimorph film is extremely thin and is made of polymer film, thereis often curl in the final bimorph product which varies from one bimorphto another, It is important to be able to register all the bimorphshutter controlling elements at the outset to insure that when all thebimorphs are in the "off" position, the shutters for each bimorphcompletely occlude all light paths, and none are in a prestressed statewhich is different from the prestressed state of the others. If this isnot the case, each bimorph will act differently in response to the samesignal.

Accordingly, a need has arisen for a bimorph light modulator for use inimplementing large scale displays which can operate at high enoughfrequency to be compatible with television signals and which can controlvery high intensity light such that the display is usable in highambient light conditions with good contrast and visibility. Further,such bimorphs must be relatively easy to assemble, and must be reliableand accurate in terms of the repeatability of the light intensitymodulation which may be achieved.

SUMMARY OF THE INVENTION

According to the teachings of the invention, there is disclosed abimorph which is easier to manufacture than bimorphs taught in the priorart. There is also disclosed a bimorph light modulator having animproved design in that resonance and chatter effects are minimized, andlight modulation response is accurate and repeatable. The bimorph lightmodulator of the preferred embodiment uses fiber optic light guides so asingle very right source may be used for input light and very large,bright, high contrast displays may be manufactured. In alternativeembodiments, other types of input and/or output light guides may also beused. Further, the device may be used in the ultraviolet or infraredspectrums, so suitable radiation guides for light at these wavelengthsmay also be used.

The bimorph is comprised of two piezoelectric films which are laminatedtogether with ultraviolet-setting glue. The film strips have apredetermined set of metalization patterns on the various surfacesthereof. These metalization patterns are registered with each other andhave unmetalized areas to eliminate dimensional changes of the filmunder the electrical contact. The metalized patterns are also formedaway from the edges of the film to eliminate the possibilities of shortsand to minimize electrostatic pinning due to edge electrostatic fields.The purpose of the metalization patterns is to allow electric fields ofthe appropriate polarity be applied across the piezo film to cause thedesired electrostrictive bending for use in modulating light. Inalternative embodiments, electrostatic pinning may be avoided by otherconfigurations of the bimorph, the substrate and the metallizationpatterns. To eliminate electrostatic pinning, it is essential that thesubstrate be conductive and that the top and bottom electrodes beconductive and connected to be at the same potential. The substrate (andtherefore the bottom metallization pattern) and the top metallizationpattern could then be at ground with the center metallization pattern athigh voltage as in the preferred embodiment. An alternative embodimentcould be used where the center electrode is kept at ground potential andthe substrate and the top electrode driven to high voltage when bendingis desired. The high voltage electrode must be etched back in whateverembodiment is used to eliminate the edge fields that cause electrostaticpinning.

The light modulator according to the teachings of the invention isconstructed using an input light guide and an output light guide. Inalternative embodiments, no output light guide is necessary. In thepreferred embodiment, both the input and the output light guides arefiber optic light guides. The output light guide has a diameter threetimes as large as the diameter of the input light guide. There is a gapbetween the output of the input light guide and the input of the outputlight guide. This gap is the path of coupling for light emerging fromthe input light guide and entering the output light guide. The lightguides are mounted on a substrate and the bimorph is mounted on thesubstrate also in a predetermined position so that it may used tomodulate the light passing through the gap. The bimorph has an aluminumfilm attached to one end thereof to act as a shutter. The bimorph ismounted on the substrate so as to register the shutter in the gapbetween the input light guide and the output light guide. The shutterregistration position is selected to completely occlude coupling oflight between the light guides when the bimorph is in its uncurvedstate. The side of the shutter facing the input light guide is coatedwith a black, light absorbing coating to eliminate light reflectionsback into the input light guide. When voltage is applied to the bimorph,the bimorph curves and the shutter is removed from the gap to allowlight coupling between the light guides.

The larger diameter of the output light guide maximizes the efficiencyof the light coupling and renders the size of the gap noncritical withina certain range.

By varying the duty cycle of the applied voltage, the duty cycle of thepresence and absence of the shutter in the gap can be varied. Thiscontrols the average light flux coupled between the light guides, andafter averaging by the human eye, the perceived effect is a modulatedaverage light intensity.

In the preferred embodiment, a bimorph of sufficient width to causeviscous air damping is used. Also, a three point mechanical mounting isused with a fulcrum in the midsection of the bimorph. This causes thebimorph to form a shallow angle with the substrate to preventhydrostatic sticking. The three point mechanical mounting is importantin obtaining high registration tolerance of the shutter with respect tothe input light guide since perfectly flat bimorphs cannot bemanufactured. Top and bottom stops are used to limit the range ofmovement of the bimorph to improve turn-on and turn-off time and to dampresonant vibrations.

Also disclosed is a process for making the bimorph and a process formounting the bimorph to eliminate stresses in the bimorph and to achieveaccurate registration.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view of one embodiment of a bimorph lightmodulator using fiber optic light guides.

FIG. 2 is a perspective view of a bimorph light modulator according tothe preferred embodiment of the invention.

FIG. 3 is a perspective exploded view of a bimorph constructionaccording to the teachings of the invention.

FIG. 4 is a drawing showing the metalization patterns for the foursurfaces of one embodiment of a bimorph constructed in accordance withthe teachings of the invention.

FIG. 5 is a flow chart of the process of manufacturing a bimorphaccording to the teachings of the invention.

FIG. 6 is a side view of one embodiment of the bimorph light modulatorat a point in the construction when the bimorph has been shutterregistered, and is ready to be fastened down for purposes of makingelectrical connections.

FIG. 7 is a flow chart of the bimorph mounting process according to theteachings of the invention.

FIG. 8 is a side elevation view of the registered position of thebimorph in a prestressed embodiment to eliminate the effects of elasticlag.

FIG. 9 is a side elevation view of another embodiment of the lightmodulator of the invention having output optics which may be attachedreadily by plugging the output optics module onto the bimorph lightmodulator shelf.

FIG. 10 is a side view of the electrical connections of the bimorphaccording to one embodiment.

FIG. 11 is a top view of an embodiment of the electrical connectionsusing conductive epoxy.

FIG. 12 is a top view of one embodiment of the shelf containing multiplelight modulators according to the teachings of the invention.

FIG. 13 is a perspective view of an embodiment of a module containing aplurality of the shelves shown in FIG. 12.

FIG. 14 is a block diagram of one type of electronic driver circuitwhich can be used in conjunction with the light modulator according tothe teachings of the invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring to FIG. 1, there is shown a perspective view of one embodimentof a bimorph light modulator according to the teachings of theinvention. An input fiber optic light guide 20 is glued to the undersideof an input fiber holder 21. In the preferred embodiment, the fiberholder 21 is 2 mil brass shimstock, and the input optical fiber 20 isglued thereto using an epoxy glue. In the preferred embodiment, theinput fiber optic light guide 20 is 250 microns in diameter. An outputlight guide 22 is registered center line to center line with the inputlight guide 20. In the preferred embodiment, the output light guide 22can be optical fiber or a bundle of optical fibers having a diameter of750 microns. The output light guide 22 may be fastened to a substrate 23in any known manner. The input light guide 20 rest on an extension ofthe substrate 23. The substrate 23 can be 1-2 millimeter (mm) brassplate or 2-3 mm plastic. In general, it is preferred to keep alldimensions of thickness as small as possible so that the maximum numberof pixels may be obtained in the Z direction. The gap between the inputfiber holder 21 and the substrate extension 25 may be left filed withair, or it may be filled with a pliable insulating material such asrubber or potting compound to provide an additional degree of mechanicalsupport.

The input light guide 20 and the output light guide 22 are separated bya gap indicated generally at 24. A shutter 26 is placed in the gap 24and is mechanically affixed to a bimorph 28. The bimorph 28 flexes inaccordance with the intensity and duty cycle of electrical fieldsapplied via a high voltage conductor 30, a high voltage center electrode31 and ground connections (not shown). When a high voltage of a properpolarity is applied, the bimorph 28 bends upward, i.e., in the positiveZ position, thereby removing the shutter 26 from the gap 24. Thispermits light in the input light guide to be captured by the outputlight guide 22 and guided to the pixel location controlled by thisparticular bimorph light modulator.

In the preferred embodiment, pulse width modulation is used for the highvoltage control signal on the electrode 30 thereby causing the shutter26 to move into and out of the gap 24 with a duty cycle controlled bythe pulse width parameter. The duty cycle controls how much of the timethe shutter 26 is in gap 24 and completely occluding light transfer fromthe input light guide 20 to the output light guide 22. The higher thepercentage of the time that the shutter 26 is in the gap 24 andoccluding light flow, the lower is the average light flux transferredfrom the input light guide 20 to the output light guide 22. This loweraverage light flux will be perceived by the human eye by virtue of theeye's natural averaging process as a lower light intensity emerging fromthe pixel controlled by the bimorph light modulator of FIG. 1.

The electrical fields needed to operate the bimorph 28 require both ahigh voltage source and a ground plane as well as metalized orconducting surfaces on the bimorph structure to cause the electricalfield to pass through the piezo film. As will become clear from thediscussion below, the ground plane must be connected to the metalizedfilms on two surfaces of the bimorph on opposite sides of the metalizedfilm used for the high voltage. The manner is which this is done will bemade clear in the discussion below. The ground plane in the embodimentof FIG. 1 is the conductive plate 32. The high voltage electrical lead30 is insulated from the conductive plate 32 by a layer of insulatingmaterial (not shown) so as to prevent a short between these twoconductors of differing potential. One ground connection is to ametalized film on the underside of the bimorph 28. Another groundconnection is to a metalized film on the top of the bimorph 28 (notshown). The exact manner of one embodiment of electrical connections isshown in FIG. 11.

Obviously, the color of the light conducted into the bimorph lightmodulator by the input light guide 20 is not important so long as theinput guide light 20 is capable of guiding that frequency of light. Thusthe bimorph light modulator of FIG. 1 can be used for implementing colordisplays by grouping three bimorph light modulators and three inputlight guides together as a single pixel where each light modulatorcontrols the intensity of one of the three primary colors. Because fiberoptic light guides are used in the preferred embodiment, distributedpixels are also possible. In such an arrangement the output fiber oroutput fiber bundle 22 is distributed in a pattern over a local matrixof red, green and blue pixel positions. In such an embodiment, the pixelpositions may overlap each other in the matrix to provide a smootherappearing display.

Referring to FIG. 2 there is shown the preferred embodiment of a bimorphlight modulator according to the teachings of the invention. The lightmodulator is built on a substrate comprised of either 1-2 mm brass or2-3 mm plastic. The substrate 23 provides mechanical rigidity for thesuperstructure built mechanical rigidity for the superstructure builtthereon. The input light guide 20 rests on the top surface of thesubstrate 23 but is mechanically affixed to a brass plate 21 which is0.002 inches thick (2 mil) in the preferred embodiment. The input lightguide 20 extends from the rear edge of the board 31 to the front edge 29of the board. As with the embodiment of FIG. 1, the input light guide 20is optical fiber of a diameter of 250 microns, and is glued to the brassplate 21, but other forms of affixation will also work.

The next layer up from the brass plate 21 is a printed circuit board 27.This printed circuit board does not extend all the way to the front edge29 of the bimorph light modulator. Instead, the printed circuit board 27extends from the rear edge 31 to a position along the axis just short ofthe bimorph fulcrum 33. The portion of the printed circuit board 27closest to the fulcrum is used to make the electrical ground planeconnection to the bimorph 28. That is, standard copper traces may beformed on the surface of the printed circuit board 27 facing theunderside of the bimorph 28 such that the bimorph may be glued to theseelectrical traces using conductive epoxy to both form an electricalconnection and a mechanical mount. The details of the electricalconnection scheme will be clarified in connection with the discussion ofFIG. 11. The mechanical attachment of the bimorph is by way of theelectrical connections to the printed circuit board 27. The electricalconnections are covered by a pliable insulating layer such as rubber orpotting compound 35.

The advantage of using the printed circuit board 27 is that theconnections to the bimorphs may be made by standard copper conductiveleads etched onto the surface of the printed circuit board. Further,integrated circuit sockets and conductive traces leading up to each pinconnection point may be formed in the printed circuit board such thatthe driver circuitry for whatever modulation scheme which is chosen bythe user may be built on the printed circuit board.

A brass spacer 37 rests of top of the printed circuit board 27 toprovide mechanical support and spacing for a top plate 39. The purposefor the top plate 39 is to shelter the bimorph 28 and to providemechanical support for a top stop 41. The purpose of the top stop 41 isto limit the upward movement of the bimorph shutter end and to damp anyresonant vibrations of the bimorph cantilevered beam. The clamping is tolimit the movement of the bimorph to a range which is adequate to causea shutter 26 to be completely removed from all possible light paths fromthe input light guide 20 to the output light guide 22 (not shown). Thebimorph vibrations, if not damped, could cause the shutter 26 tooscillate up and down in the Z direction each time the bimorph 28 isrelaxed to place the shutter 26 in front of the input light guide 20,i.e., in the off position. These vibrations could cause the shutter 26to move in and out of the light path when it is supposed to be solidlyin the light path and completely occluding light transfer from input tooutput. Such vibrations are a cause of error in controlling theintensity of light by a cantilevered beam bimorph and destroy therepeatability of the result. The top and bottom stops could be knifeedge stops. In such an alternative embodiment, the voltage drivingsignal can be reversed just before contact of said bimorph with eitherstop to decelerate the bimorph and eliminate the bounce which wouldotherwise occur when the bimorph hits the stop where no viscous dampingis present because of the narrow nature of the top and bottom stops.

To improve the speed with which the bimorph opens and closes, thebimorph is "overdriven" in the sense that the amount of voltage appliedis much greater than the amount of voltage needed to move the bimorphbetween the top and bottom stops. This reduces the rise and fall timesof the operation of the gate in moving from an open to closed positionand vice versa as would be seen if shutter position as a function oftime were plotted at the voltages used in the invention versus lowervoltage driving signals.

Conductive traces 43 and 45 represent high voltage signal lines toadjacent bimorphs. The structure shown in FIG. 2 is typically built with16 bimorphs side by side such that 16 pixels in black and white displaysmay be controlled with the structure of FIG. 2. When these structuresare stacked, a two-dimensional array may be formed.

Referring to FIG. 3 there is shown a perspective, exploded view of atypical bimorph construction. A bimorph is a laminated structure using aclass of polymers that exhibits piezo electric properties. The mostcommon polymer of this class of materials is poly(vinylidene fluoride)₂,sometimes referred to as PVF₂ or PVDF. This class of polymers can bemanufactured in such a way that the molecular structure is aligned. Thenthe structure is poled in an intense electric field to effect separationof charges resulting in an electret material which exhibitspiezoelectric properties and electrostrictive properties.Piezoelectricity is the property that when a material is subjected tomechanical stress, it generates a voltage. Electrostrictive effect isthe opposite; when the material is subjected to an electric field, thematerial changes dimensions. The manufacture of such films is a wellknown process, and it is commercially available under the trademarkKynar™ from Pennwalt Corporation in Philadelphia, Pennsylvania. Suchfilms come with metalization coatings on the surfaces which can be usedto apply the electric fields by applying voltages to these metalizationfilms. A strip of PVF₂ will lengthen (or shorten) by a fractional amountthat increases with voltage and decreases with film thickness.

Bimorphs are constructed by fastening two such films together in such away that the electric field across each is arranged to contract onelayer and elongate the other. This bimorph construction will besometimes referred to herein as a bimorph beam or a beam. With one endof the beam held fixed and a voltage applied, the free tip of the beamwill undergo an excursion or tip deflection. With a shutter such as theshutter 26 in FIG. 1 attached to the free end of the beam, this tipdeflection allows a bimorph gate to open or close a light path therebyallowing light coupling and decoupling between an input light guide andan output light guide.

Although bimorph beams are not new, the structure of the light modulatordescribed herein is new in that the bimorph is constructed using verythin films with the resonance and chatter problems reported by earlierinvestigators eliminated and with a very accurate registration of theposition of the shutter end of the bimorph to the light path in a way toeliminate any unintended stresses in the beam so that all bimorphs actsimilarly to the same signal. Further, the bimorph is easier toconstruct, more reliable and less susceptible to shorting. The bimorphlight modulator uses fiber optic light guides so certain advantages areachieved. First, the fibers can be grouped into bundles so a singlelight source may be used to supply input light to all bimorphs. Thislight can be guided to pixel positions in a very large display since thefibers are flexible. This eliminates the complexities of trying to dothe same thing with discrete, classical optics. Only one input fiber isneeded for each gate. but multiple output fibers can be used for eachgate.

The construction details of the bimorph cantilevered beam are asfollows. The bimorph is made of two strips 34 and 36 of piezoelectricfilm. In the preferred embodiment, these film strips are 9 microns inthickness along the Z axis. The two films 34 and 36 are laminatedtogether by a glue layer 38 to form a laminated beam construction. Inthe preferred embodiment, the thickness of the glue layer isapproximately one half the film thickness or less than 5 microns. Thefour surfaces of the two piezoelectric film strips are labeled S1, S2,S3 and S4 in FIG. 3. These same surface numbers are used in FIG. 4 whichshows the metalization patterns which are formed on each surface. Theglue layer 38 extends from the edge of surface S2 to the end of thebimorph having the most positive Y axis coordinate. This most positive Yaxis coordinate end of the beam will become the end on which the shutteris attached later in the process of manufacturing the light modulator.The glue layer 38 covers all of surface S2 and the portion of surface S3which is covered by the surface S2 in the final construction. Theparticular glue used should have approximately the same stiffness orYoung's modulus when set as the piezo film itself. Further, the glueshould set in ultraviolet light in the preferred embodiment.

Although it is possible to use epoxy glue or other glues, such glues arenot preferred since they give a limited amount of time to work with thefilms after the glue is applied and before the glue sets. It isdesirable to be able to slide one film on top of the other so as toprecisely register the two film strips with relation to each otherbefore the glue sets. Further, it is desirable to not have any wrinkles,bubbles, or other discontinuities in the surface after the glue has set.Therefore, it is convenient to have a glue which sets in ultravioletlight so that the construction of the bimorph may proceed in anunhurried fashion and so that wrinkles and bubbles may be eliminatedfrom the laminate before the glue sets. With ultraviolet setting glue,all steps of the construction process may be carried out at leisure, andwhen completed, the glue may be set by exposing the structure toultraviolet light.

It is important to get a uniform thickness for the glue layer, and it isimportant to be able to control the thickness of the glue layer so thatthe glue layer is as thin as possible. Thick glue layers cause thebimorph beam to be overly stiff. This excessive stiffness will limit therange of deflection of the shutter when voltage is applied. It isnecessary that the bimorph be flexible and mobile enough to completelyremove the shutter from the light path. In the preferred embodiment, theglue used is NOA-81 manufactured by Norland Optical Products.

A key factor in determining the glue thickness is how much force isapplied during the step of setting the glue. The process of gluing thetwo film strips 34 and 36 together is as follows. This process will bedescribed with reference to FIG. 5 which is a flow chart of the steps inthe process. The first step is symbolized by block 51 which representsthe process of laying out a sheet of the piezoelectric film which willbecome, after dicing, a plurality of the bottom strips 34. This sheethas the metalization patterns for surfaces S3 and S4 shown in FIG. 4formed on it in side by side relationship with the surface S3 patternson the top side of the film and the surface S4 patterns formed on theunderside of the film in registration with the S3 surface patterns asshown in FIG. 4.

The long dimension of the metalization patterns is formed coincidentwith the access of highest electrostrictive sensitivity. That is, in theprocess in which the film is manufactured, the piezoelectric propertiesare introduced into the sheets of film by a process known as poling. Inthis process a strong electric field is applied across the thickness ofthe film at 110° C. to align the polymers into polarized chains. Thisproduces polarized film with a positive and a negative side. Theextrusion process also introduces an asymmetry into the film. This meansthat the polymer chains in addition to being polarized through thethickness of the piezo film, are stretched in the direction ofextrusion, and this direction becomes the direction of the highestpiezoelectric or electrostrictive sensitivity. This direction is alsothe direction chosen for the longitudinal axis of the metalizationpatterns.

As indicated by step 5, the bottom sheet of piezo film is laid upon aglue resistant surface. In the preferred embodiment, as it is presentlyknown, the bottom sheet is laid upon a heavy glass plate with a film ofSaran wrap or other glue resistant film interposed between the piezofilm and the glass surface. In other embodiments, the glue resistantsurface may be Teflon or some sprayed material. The purpose of the glueresistant surface is to prevent the bimorph from becoming glued to theglass plate during the process of setting the glue. Ultimately thebimorph laminated construction is to be squeezed between two glassplates so as to make the glue layer 38 as thin as possible. Thissqueezing process squeezes the glue out from between the two piezo filmsheets. This excess glue can glue the bimorphs to the glass if the glasssurfaces are not made glue resistant.

The next step is to eliminate wrinkles in the bottom film as symbolizedby step 53. Because the piezo films are so thin, they exhibit a naturalcurl resulting from the process by which they are made and are somewhatdifficult to work with. Steps must be taken to eliminate this curl. Oneof these steps is annealing the film after metalization by temperaturecycling it between 20° C. and 70° C. several times. Any remaining curlafter this annealing process should be eliminated prior to laminatingthe two films together so as to produce as straight a bimorph aspossible. Thus some step must be taken when laying the bottom sheet onthe glass plate to eliminate any curl which would cause the film to notlie flat on the glass surface. One way of doing this is to spray theglass surface with a wetting agent which evaporates such as Windex andthen to lay the piezo film on the glass in contact with the wettingagent. The wetting agent tends to make the thin film stick to the flatglass surface and lie flat upon it. The film can then be freely slidacross the surface to register it with registration marks. Theseregistration marks insure that the film is in the proper location forlamination with the top film. This registration is necessary because thetop film will be brought into contact with the bottom layer of filmusing a jig having guide means for a top plate to which the top film isattached by wetting agent or some other means. Typically, the glassplate upon which the bottom film is placed is registered in a fixedposition by a jig or other alignment means. The registering of thebottom film to registration marks on the bottom glass plate is onlynecessary for automated manufacturing processes where a machine willlower the top film onto the bottom film. If hand placement is beingused, this registration may be omitted since the operator may visuallyalign the top film with the bottom film to achieve the alignment shownin FIG. 3. Note that the top film 36 is shorter that the bottom film 34in FIG. 3 and that the top film is aligned with the bottom film suchthat the shutter ends coincide. This leaves the opposite end of the topfilm 36 short by the difference in film lengths from aligning with theend of the film strip 34 opposite the shutter. This allows space for anelectrical connection to the metalization pattern on surface S3.

Alternative ways of eliminating wrinkles would be electrostatic chargingof a conductive pattern on the bottom glass plate with opposite chargingof the metalization pattern on the bottom film so as to cause the twosurfaces to cling together electrostatically. Another way of eliminationwrinkles would be to use a suction plate having tiny openings in thesurface thereof instead of the glass plate. The tiny openings in theflat surface of the suction plate would be coupled to a vacuum manifoldsuch that the bottom film could be maintained in flat relationship tothe suction plate by application of vacuum to the vacuum manifold. Ifthe Windex or wetting agent method is used, the wrinkles may beeliminated by rolling the film onto the flat surface using a roller.This process eliminates wrinkles and does not induce stresses in thefilm which could be produced by stretching the film to eliminate thewrinkles. It is important not to induce stresses in the films becausesuch stresses will result in curled or twisted bimorphs after cutting.

The next step is to apply the glue to the surface S3 of the bottom film34. Preferably, this glue application process will be by some methodwhich does not induce stresses in the bottom film 34. Preferably, theglue is sprayed on, but it may also be dabbed on gently. Alternatively,the glue may be applied to the top film 36 of the surface that will facethe bottom film 34.

Next, the top piezo film 36 is placed on glue resistant surface number 2as symbolized by block 57. Typically, this glue resistant surface is aglass plate with plastic film or some other glue resistant coatingbetween the piezo film and the glass. This process is the same as theprocess described with reference to step 51. Next, step 59 is performedto eliminate the wrinkles and register the top sheet with alignmentmarks on the glue resistant surface number 2. The process of step 59 isthe same as the process of step 53.

An alternative method to steps 57 and 59 is to hand register the topsheet with the bottom sheet of piezo film. Since, in the preferredembodiment, the glue is UV setting, this hand registration process canbe formed at a leisurely pace to insure that the registration is properand that no wrinkles exist in the final laminated structure prior to theapplication of ultraviolet light. The preferred method of constructionof the bimorph beam, however, is automated.

The next step in this process of automated construction of the bimorphbeam is to bring the glue resistant surface number 2 straight down ontoglue resistant surface number 1 in aligned relationship to mate themetalized patterns as shown in FIG. 3 and 4. In the preferredembodiment, this is done with a machine. However, it may also be done byhand in a jig where the top and bottom glue resistant surfaces are glassplates having glue resistant coatings thereon. These two glass platesare of the same size and may be registered with corner registrationguides such that the top glass plate may be lowered gently down upon thebottom glass plate in registered alignment with the corner alignmentguides. The ultimate goal is to achieve the alignment shown in FIG. 3,and any method of achieving this alignment will suffice for purposes ofpracticing the invention.

Next, clamping to get the proper glue thickness must be performed assymbolized by step 63 in FIG. 5. There, surface 1 and surface 2 of thetop and bottom pressure plates are clamped together with sufficientforce, evenly applied over the surfaces, to squeeze the piezo filmstogether sufficiently to achieve the desired glue layer thickness forlayer 38 in FIG. 3. The glue layer thickness is a function of the amountof force applied. This amount of force will have to be experimentallydetermined by the user. Step 63 is the last step if the glue used is anonultraviolet setting glue.

If the glue used is an ultraviolet setting glue, then the final step inthe process is step 65 where the clamped structure is exposed toultraviolet light for a sufficient time to set the glue. In thepreferred embodiment, the time needed is approximately two hours using astandard 15 watt black light. The relative long time to set the glue iscaused by the existence of the metalization film on the surfaces S1 andS4. This metalization filters out large amounts of the ultraviolet lightthereby slowing the process of setting of the glue layer 38 betweenthese two surfaces. Further, if ultraviolet setting glue is used, theglass pressure plates and glue resistant films used should be ofmaterials which do not block the particular ultraviolet lightwavelengths needed to set the glue. Further, the clamps used should notbe placed in a location which would block the passage of ultravioletlight to the glue layer 38.

Referring to FIG. 4, more detail on the metalization patterns is shown.The four rectangular figures in FIG. 4 represent the four surfaces S1through S4 shown for the bimorph laminate of FIG. 3. The metalizationpatterns in the preferred embodiment are sputtered gold with a thicknessof 350 angstroms. Other metalization materials may be used such ascopper provided that the metalization material will form a good bondwith the particular type of electrical connection material chosen. Inthe preferred embodiment, this electrical connection material isconductive epoxy, but in alternative embodiments, a low temperatureeutectic such as a mixture of bismuth and indium is used to sweat smallcopper pads approximately two millimeters on a side to the metalizationpattern. Prior to soldering the copper pads onto the metalizationpatterns, fine wires are soldered to the copper pads using hightemperature solder. The bimorph beams are extremely sensitive to hightemperatures, and can be destroyed by being subjected to temperaturesgreater than 70°-80° C. either during the construction process or duringoperation. Further, the bimorphs exhibit undesirable thermal curl whichcauses them to distort at high temperatures unless the thermal curl canbe completely eliminated by the annealing process described above.

In FIG. 4 the metalization pattern is shown as uncrosshatched area whereunmetalized film is shown as hatched area. For example, on surface S4,an unmetalized area 46 is shown which corresponds generally to the sizeand location of the high voltage connection pad 48 on surface S3. Thepurpose of the unmetalized portion 46 on surface S4 is to eliminate theelectrostrictive action of the film in this region by eliminating theground plane contact in this region. The metalization pattern in S3 isthe high voltage electrode whereas the metalization pattern on surfaceS4 is the ground electrode. Electrostrictive behavior of the bottom filmstrip 34 is induced by charging the metalization pattern on surface S3to a high voltage of approximately 200 volts. The surface S4 isconnected to ground potential so that a high strength electric fieldpasses through the bottom piezo film 34. Because there is no groundplane contact lying under the high voltage contact pad area 48, noelectrostrictive behavior occurs in this region. The purpose ofeliminating electrostrictive behavior in the vicinity of the highvoltage contact pad 48 is to improve the lifetime, reliability andmechanical integrity in general of the high voltage contact in the area48. If the dimensions of the surface of the area 48 were changing underthe electrical contact, the integrity of that contact would becompromised, and the contact could eventually fail.

The unmetalized region around the edges of surface S3 is present toimprove the electrical integrity of the high voltage contact inpreventing shorts between the high voltage metalization pattern onsurface S3 and the ground plane connected to the metalization pattern onthe surface S4. Since the piezo film 34 is only 9 microns thick, if theetched back region on surface 53 were not present, only 9 microns ofpolymer film would exist between the high voltage contact and the groundplane. This could cause shorts between the high voltage contact and theground plane if dirt, glue or other contaminating materials caused abridge between the ground plane and the high voltage electrode. Theunmetalized region around the edges of the surface S3 minimizes thispossibility. Further, this unmetalized region also reduces thepossibility of electrostatic pinning caused by edge fields. If themetalization pattern on surface S3 were to extend all the way to theedges, and electrical field between the high voltage electrode and theground plane 9 microns away would exist in the edge space around theedges of the bottom piezo film 34. Because opposite charges attract byelectrostatic force, and the ground plane could be considered anopposite charge, there would be an electrostatic attraction caused bythe edge field. The force exerted by this edge field attraction couldmake it more difficult for the bimorph to lift itself up and away fromthe ground plane when the shutter is to be withdrawn from the lightpath.

The surface S2 is unmetalized in the preferred embodiment since it isonly necessary to generate an electric field through the top piezo film36. This is accomplished by the presence of the metalization pattern onthe surface S3 and the ground plane metalization pattern on the surfaceS1 of the top sheet 36. In alternative embodiments, the surface S2 couldbe metalized in a similar fashion to the surface S3 if an electricalconnection for high voltage could be made to this surface. This could bea preferable construction because it would cause the intensity of theelectric field through the piezo film 36 to be the same as the intensityof the electric field through the piezo film 34. As the bimorph ispresently constructed, the electric field through the piezo film 36 issomewhat attenuated by the presence of an additional 5 microns ofspacing between the high voltage electrode and the S1 surface groundplane electrode caused by the presence of the glue layer 38. However,the inconvenience of making an electrical contact to a metalizationpattern on the surface S2 is judged to far outweigh the advantage ofhaving the electric field intensities identical in the films 34 and 36.One way of making this electrical connection is to use conductive epoxyto bond the two piezo films together to form the bimorph.

The unmetalized region 50 on the surface S1 is intended to preventshorts between the ground plane electrode on the surface S1 and the highvoltage electrode on the surface S3. The edge 52 of the top strip 36will coincide with the dotted line of the high voltage pad boundaryshown on the surface S3. Thus only 9 microns of piezo film will separatethe high voltage electrode from the top ground plane electrode, andshorts could easily occur if dust, glue or other contaminating materialshappen to simultaneously contact the two electrodes.

In the preferred embodiment, the metalization pattern shown in FIG. 4 isformed by sputtering gold onto the piezo film. However, the sputteringprocess for gold causes high temperature due to the energy needed tosputter the heavy gold atoms off the target. Thus, in alternativeembodiments, other metalization patterns may be used such as coppersince lower energies can be used to sputter copper. Aluminum will notwork however since it is too difficult to bond to aluminum through thealuminum oxide unless some steps are taken to eliminate the oxide. Thereis a very high current density at the locations of the electricalconnections. If the electrical connection is not of a low resistivity,failure can occur caused be heating in the area of the contacts, arcingor other such phenomena. It is important however that the metalizationpattern have low resistance and be free of pinholes, stretches and otherdefects which could raise the resistance at the area of the defect. Thereason for this is that high current densities will exist in themetalization films during changes of voltage levels during normaloperation. The gold films separated by insulating material togethercomprise a capacitor. This capacitor is being driven by fast rise timesquare wave high voltage signals to get rapid turn on and turn offtimes. Each time the high voltage changes voltage level, a high currentdensity results as the capacitor charges or discharges. If a localizeddefect raises the resistance in a particular area, arcing can occur inthat area. This arcing can eventually cause the defect to become an opencircuit thereby destroying the utility of the bimorph.

The unmetalized portions of the surfaces S1 through S4 can be achievedeither by etching or by masking during the metalization process. Analternative embodiment is to blow the gold off the bimorph edges in theareas to be unmetallized using a ultraviolet light laser. Because theprocess of etching the gold film using standard photolithographictechniques is messy, somewhat inaccurate and slow, and because itcreates waste disposal problems for the used etchants, it is preferredto use masking techniques during the metalization process. In such aprocess, the unmetalized areas are masked off using standardphotolithographic and masking techniques prior to the sputtering of themetal coating. When the masking material is removed, the desiredmetalized and unmetalized areas for each bimorph surface will bedefined. The etching process works perfectly well, however, and theaccuracy of the etching need not be very high. An RMS variation of edgestraightness on the order of 25 microns is acceptable.

Referring to FIG. 6 there is shown the bimorph light modulator in a sideview at a point in the process of constructing the light modulator afterregistration has been completed. The mounting and registration processis important, because it eliminates prestresses in the bimorphcantilevered beam and insures that the offstate of the bimorph will bein a position such that the shutter 26 completely blocks all light pathsfrom the input light guide 20 to the output light guide 22.

There are four requirements for the mounting and registration process.First, the attachment must be secure and long-lived. Second, the shutterend of the bimorph should be registered such that the lowest tip of theshutter, i.e., the point on the shutter having the lowest Z axiscoordinate, should be registered to within 25 microns of the point onthe perimeter of the input fiber 20 having the lowest Z coordinate.Further, the shutter 26 should be registered to within 25 microns alongthe X axis from the output end of the input light guide 20. The bottomedge of the shutter must be below the lowest edge of the input fiberwith a minimum overhang of 50 microns. The third requirement of theregistration and mounting process is that the bimorph should make asmall angle with the substrate surface 21 in order to beneficially usethe viscous damping effect of the air advantageously to damp resonanceand chatter while simultaneously avoiding or lessening the effect ofhydrostatic sticking of the bimorph to the surface 21 of the substrate.This hydrostatic sticking would occur if the bimorph were allowed tocome into contact with the surface 21 along the whole length of thebimorph. A further requirement of the mounting process is that when thebimorph is registered, no stresses should exist in the bimorph otherthan those intended to be there, and all bimorphs should have the samelevel of prestress.

FIG. 7 is a flow diagram of the process of mounting and registering thebimorph which achieves the above stated objectives. With concurrentreference to FIGS. 7 and 6, this process will now be explained. Thefirst step is represented by block 67 where a hemispherical fulcrum 69is glued to the bimorph. The size of the fulcrum and the location whereit is attached to the bimorph are selected so that the bimorph makes ashallow angle with the top surface 21 of the input light guide holder27. Typically this shallow angle is about 5°, but other angles willwork. The purpose of this shallow angle is to allow the air to be usedfor cushioning or viscous damping of the landing of the bimorph on theedge 60 of the input light guide holder 27 when the shutter 26 islowered into the gap 24. Because the bimorph 28 is approximately 3-4 mmwide, and is very light, the air between the bimorph 28 and the surface21 tends to cushion the landing of the bimorph on the edge 60 as the air"mushes" out of the way when the bimorph 28 is landing. This dampingeffect tends to reduce the bounce of the bimorph 28 up and away from theedge 60 in the positive Z direction which would occur if the bimorph 28were to land on the edge 60 in a vacuum. Further, the damping effect ofthe air tends to minimize resonance vibration which will occur if thenatural mechanical resonance of the cantilevered beam is excited by thedriving signal. Such bounce and resonant vibration will destroy thereproducibility of the bimorph response to a given driving signal sincethe chatter and resonance tends to alter the amount of light fluxcoupled between the input light guide 20 and the output light guide 22and because the bounce and resonant vibration may not be the same eachtime the same input signal is applied. The shallow angle also minimizesthe hydrostatic "pinning" effect which would otherwise occur if thebimorph 28 were allowed to lie flat on the surface 21. The reader canvisualize this hydrostatic sticking effect by trying to pick up a pieceof paper by its edge which is lying flat on a glass surface. Thishydrostatic effect is caused by the evacuation of much of the airbetween the bimorph 28 and the surface 21 when the bimorph 28 is allowedto settle flat onto the surface 21. When an attempt is made to raise thebimorph away from the surface 21, a temporarily lower air pressure wouldexist in the space between the bimorph and the surface 21 until air canrush in from the sides to equalize the pressure. During the time whenthe air is rushing in from the side before the air pressure on the topand bottom of the bimorph is equalized, there exists a pressuredifferential. This differential exists because there is higher airpressure on top of the bimorph than underneath it. This pressuredifferential tends to resist movement of the bimorph 28 up and away fromthe surface 21. By establishing the fulcrum 69 so that the shallow angleis formed between the bimorph 28 and the surface 21, evacuation of airfrom the space therebetween is eliminated so that the hydrostaticpinning effect cannot occur. The proper shallow angle is achieved byusing a hemispherical fulcrum which has a diameter of 1.5 mm and whichis located at the proper point on the bimorph to establish the angle.

The point of attachment of the fulcrum to the bimorph is also selectedto be left of the center of gravity of the bimorph such that the shutterend to the right of the fulcrum 69 in FIG. 6 is heavier than the portionof the bimorph to the left of the fulcrum. This imbalance aids in theregistration process as will become clear from the discussion below.

These substeps in gluing the fulcrum to the bimorph symbolized by step67 in FIG. 7 are shown to the right of the block 67 in FIG. 7 asindicated by the dashed leader line 71. The first substep is step 73wherein a layer of glue of the proper thickness is established. Theproper thickness may be experimentally determined with the criterionbeing that the amount of glue used to glue the fulcrum to the bimorphshould not be so excessive that it becomes enough to cover the fulcrumand should not be so little as to render the attachment of the fulcrumnot mechanically secure. A good way of establishing the proper thicknessfor the glue layer is to fasten two metal shims or strips of the desiredglue layer thickness to a glass substrate and then fill the slot betweenthe two shims with glue. A squeegee or other straight edge may then bedragged over the top of the two strips leaving a layer of glue of thedesired thickness between the two strips.

The next substep is step 75 wherein a dab of the glue from the gluelayer made in substep 73 is picked up using a pickup tool. The desiredthickness of the glue drop so picked up will be established by thethickness of the layer of glue established in substep 73 while thediameter of the drop will be established by the diameter of the tip ofthe pickup tool. Typically, the pickup tool is a small truncated conewith the truncation at the tip of the cone at the point of the desireddiameter for the glue drop. The pickup tool is simply lowered into theglue layer and picked up thereby leaving a drop of glue at the tip ofthe pickup tool by the adhesion of the glue to the pickup tool tip.

The next step, symbolized by block 77, is to place the drop of glue atthe proper location on the bimorph. In the preferred embodiment, this isaccomplished by a jig. The bimorph is placed on a flat surface of thejig and registered with registration marks. Vertical guide assembliesare provided at locations relative to these registration marks such thatthe pickup tool may be slid down these vertical guides in such a mannerthat the tip of the pickup tool will land on the bimorph at the preciselocation where the fulcrum is to be attached. The drop of glue on thetip of the pickup tool will then be deposited at the proper location onthe bimorph.

Next, a fulcrum is picked up by a fulcrum placement tool. In thepreferred embodiment, the fulcrum is picked up by a vacuum tool whichhas a tip which mates with the curved surface of the fulcrum. The tiphas a vacuum channel in it connected to a vacuum pump such that thefulcrum may be retained on the tip by suction through application ofvacuum to the vacuum channel. Alternative methods of picking up thefulcrum could include magnetic attraction or electrostatic attraction.

Next the fulcrum and the pickup tool are lowered so as to place thefulcrum in the glue drop at the proper point on the bimorph. This stepis accomplished in the preferred embodiment using the same jig as wasused to register the drop of glue at the proper location. That is, thepickup tool is lowered down the vertical registration guides such thatthe tip of the pickup tool automatically arrives over the location ofthe glue drop. The fulcrum is then lowered into the glue drop and thevacuum to the pickup tool is cut off so that the pickup tool may beremoved and the fulcrum will stay in the glue drop. The final step is toallow the glue to set as symbolized by block 83.

The next step is mounting the bimorph and registering it is to place theultraviolet setting glue drops on the surface 21. This step issymbolized by the block 85 is FIG. 7. These glue drops will securelymechanically attach the fulcrums 69 to the surface 21 for the bimorphson each particular shelf such as that shown in FIG. 2. The glue dropsare ultraviolet setting in the preferred embodiment and are sized suchthat they will form a secure mechanical attachment while not being solarge as to cause difficulties in other steps of the process. The sizeof the glue drops can be controlled in a similar fashion as was done insubsteps 73 and 75 previously discussed. The location of the glue dropson the surface 21 can be accomplished in a similar fashion as discussedabove with reference to step 77. A different jig may be used in whichthe structure of FIG. 6 is registered such that the vertical guides willcorrectly position the glue drops on the surface 21. It is also possibleto use the same jig as was used to glue the fulcrums to the bimorphs andto use the same vertical registration guides.

The process of attaching the shutter is to glue a strip of foil to theshutter end of the bimorph. The length of the shutter is then cut byregistering the bimorph in a jig and using a cutting tool registered toguides on the jig to register the cutting position of the blade on thefoil. A slight force is then applied to the cutting tool to cut throughthe foil. The bimorphs are then clamped down by another tool such as aplastic block which is registered on the jig to clamp all or part of thebimorph on the film side of the shutter holding edge down to a flatsurface while exposing the film. The clamping tool has a surface alignedwith the shutter edge which is perpendicular to the surface of thebimorph and defines a plane in which the line defined by the edge of thebimorph lies. A sponge is then used to lift the foil up and bend it atthe edge of the bimorph so as to bend the foil to lie on theperpendicular surface defined next above. This bends the shutter to theproper angle. This process is done before the fulcrums are lowered intotheir respective glue drops.

The next step is to allow the bimorphs to free fall into contact withthe edge 60 such that the shutter 26 is in the gap 24. This may be doneby hand placement or in automated fashion. In either process, after thefulcrums are placed in their respective glue drops, the bimorphs arereleased from a position with their shutter ends held above the bottomstop surface 60 such that the shutter ends fall down into contact withthe edge 60. A vacuum, electrostatic or other type of tool may be usedto retain the bimorphs 28 in the position above the bottom stop untilthe free fall is triggered at which time the force holding the bimorphabove the bottom stop is released so that gravity takes the bimorph downto the bottom stop.

In the preferred embodiment, a registration tool having vacuum ports isused to maintain the bimorphs 28 in a raised position by suction forcesprior to the free fall. This same tool is used to hold the bimorphs inthe proper position to allow the fulcrums to be lowered into thecorresponding glue drops. The bimorphs and the tool are then lowereddown vertical registration guides until the fulcrums 69 are located inthe glue drops on the surface 21. The shutter ends 62 of the bimorphswill be in a raised position such that the shutter 26 is not in the gap24 at the point when the fulcrums 69 contact the glue drops on thesurface 21.

The free fall portion of the registration process is then performed. Toaccomplish this in the preferred embodiment, the vacuum is released suchthat the bimorphs fall into their positions in contact with the edge 60.Because the fulcrums 69 are hemispherical in shape and because therounded surfaces face the surface 21, during the free fall intoregistered position, any cant of the shutter edge 62 at an angle to theY axis may be eliminated during the fall. To ensure that the shutteredge 62 is parallel to the Y axis prior to setting of the glue drops onthe surface 21, an alignment tool having flexible projection fingers 89is used. This tool is brought close enough to the edges 62 of thebimorph such that one finger 89 comes into contact with each bimorphshutter edge 62. The flexible fingers 89 are elastic but somewhatmalleable so that each finger may be aligned with the center line of thebimorph to which it corresponds. Each finger is elastic enough to applya slight downward pressure in the direction of the negative Z axis tothe edge 62. Because the edge 60 is parallel to the Y axis, the slightdownward pressure will force the edge 62 to register itself parallel toedge 60 and to the Y axis. The resultant structure is then exposed toultraviolet light to set the glue drops on the surface 21 therebymechanically securing the fulcrum 69 is registered position. Theselatter two steps of applying slight downward pressure and then exposingthe glue drops to ultraviolet light are symbolized by blocks 89 and 91in FIG. 7.

Note that the process of free fall registration allows each bimorph tosettle to a registered position without imposing stresses on the bimorphfilm in the registration process itself. It such prestresses were fixedinto the bimorphs during the registration process and the glue drops onthe surface 21 were then set with each bimorph having differentprestress loads, each bimorph would respond differently to the sameinput signal, i.e., each bimorph would begin to open at a slightlydifferent voltage. This would cause undesirable variation from one pixelto another for a uniform input signal. The registration process isimportant, as is the elimination of as much unintended prestress aspossible in the bimorph structure so as to insure that all the bimorphsbegin to open at approximately the same voltage. That is, all thebimorphs in a particular application should start their upward movementto withdraw the shutter 26 from the gap 24 at approximately the samevoltage.

Some mechanical prestressing of the bimorphs is desirable, becausebimorphs exhibit a phenomenon known as elastic lag. The effect of thisproperty is to distort the response of the bimorph when driven by apulse-width modulated electrical signal. This distortion takes the formof a D.C. bias which is introduced into the optical response of thebimorph gate. In order to avoid this problem and to insure repeatabilityin the response, mechanical prestressing of the bimorphs is necessary.

The elastic lag which causes this problem is the result of theproperties of the bimorph film. When the bimorph is subjected to avoltage to cause it to bend upward, the shutter 26 will be removed fromthe gap 24. When this voltage is released, the bimorph straightens outand simultaneously, the shutter end 62 begins to fall so as to lower theshutter 26 back into the gap 24. However, as the bimorph straightensout, it does not return in a continuous fashion to the registered stateshown in FIG. 6. Instead, the bimorph straightens out to a pointsomewhere above the registered position, i.e., some point in thepositive Z direction above edge 60. From that point, the rate of descentof the bimorph changes to a much slower rate due to the effect ofelastic lag. Since this adversely affects the amount of time needed to"close" the bimorph such that the shutter completely occludes the lightpath, the elastic lag shows up as a D.C. bias in the optical response ofthe bimorph gate.

The mechanical prestressing solution to this problem is to place shimsunderneath the input light guide 20 and the output light guide 22 so asto raise both light guides and the edge 60 of the bottom stop to ahigher point on the Z axis. Alternatively, the position of the bottomstop registration edge 60 may be raised and the light guides left wherethey are if a longer shutter is used so that the light path iscompletely occluded when the bimorph is resting on the edge 60. This newpoint for the edge 60 is selected to be the point where the elastic lageffect starts Thus the movement of the bimorph downward is stopped bythe edge 60 at the point where the elastic lag would normally takeeffect. In some embodiments where the driving frequency of the bimorphgate is not high enough to make the elastic lag a problem, these shimsmay be omitted.

The shimmed structure is shown in FIG. 8 with shims 91 and 93 raisingthe input light guide 20 and the output light guide 22 by an amountequal to the distance covered by the elastic lag. Thus the top edge ofthe bottom stop, i.e., edge 60 is at the point where the elastic lagstarts in the downward movement of the bimorph 28 in the negative Zdirection.

Referring to FIG. 9, there is shown a side view of the bimorphconstruction using a top stop. In the structure of FIG. 9, the brassshim stock 27 in FIG. 6 is augmented by a five mil layer of double sidedfoam tape 91. Prebiasing in FIG. 9 is obtained by inserting additionalshims between the fiber 20 and the tape 91 after mounting of the bimorphis done. Additional shims 93 are then placed under the output lightguide to maintain the alignment of the light paths of the two lightguides. FIG. 9 also shows an alignment pin and hole combination 95 and97, respectively, which are used to align the substrate 23 supportingthe bimorph to an adjoining substrate 99, which supports the outputlight guide 22. This alignment pin and hole combination insures that thecenter lines of the two light guides 20 and 22 are coincident. In analternative embodiment, the pin/hole combination 95 and 97 are replacedwith a pair of grooves in the top and bottom surfaces 92 and 94. Thesegrooves (not shown) are engaged by projections (not shown) on thesubstrate 99 and the top piece 96 coupled to the substrate 99. Theseprojecting portions engage the grooves so that the "exit optics", i.e.,the substrate 99 and accompanying structures, may be plugged orunplugged easily. After the grooves are engaged by the exit optics, thesubstrate 99 may be slid along the Y axis until the centerlines of thelight guides are aligned.

FIG. 9 also shows a top stop 101. The top stop 101 is a projectingportion of a cover piece which is mechanically affixed to a top-printedcircuit 103. The cover serves to protect the bimorphs 28 from anyphysical intrusions by gusts of air or physical objects. This top stop101 is placed in the path of movement of the bimorph 28 to limit themaximum amount of upward movement that the bimorph 28 may make. The topstop 101 is placed at a position on the Z axis such that the shutter endof the bimorph 28 may move upward in the positive Z direction onlyenough to remove the shutter 26 from the gap 24 far enough to clear alllight paths from the input light guide to the output light guide. Whenthe bimorph 28 nears the top stop 101, any resonant ringing in thecantilevered beam is also eliminated by viscous damping. The positioningand size of the fulcrum 69 is FIG. 9 are as described earlier withreference to FIG. 6, and the width of the bimorph 28 is such as to causeviscous air damping to occur to minimize bounce or chatter when thebimorph 28 lands on the bottom stop 60. The same viscous air dampingoccurs when the bimorph comes into contact with the top stop if the topstop is at least one third the length of the bimorph from the shutterend to the fulcrum position. The cavity 102 is useful in that it allowsclearance for any curves in the bimorph when the bimorph is raised to aposition to be in contact with the top stop.

The printed circuit board 103 lies above the bimorph 28 on the Z axis,and is one of a pair of printed circuit boards used in the embodiment ofFIG. 9. The other printed circuit board 107 lies beneath the bimorph.The purpose of these two printed circuit boards is to allow electricalconnections to be made to the two metalized surfaces which act as groundplanes for the bimorph and to the single metalized surface which acts asthe high voltage electrode of the bimorph. The ground connections tosurfaces S1 and S4 of the bimorph are shown at 104 and 105,respectively. The ground connection 104 between the top surface S1 ofthe bimorph and a metalized conductive pattern on the printed circuitboard 103 may be made by a low temperature eutectic solder or,preferably, by a conductive epoxy. The ground connection 105 couples themetalized pattern on the surface S4 of the bimorph to a metalizedconductive layer on the top surface of the printed circuit board 107.The metalized conductive pattern electrically connected to theelectrical contact 104 makes contact with a ground wire in a ribboncable 109 which has multiple wires connected to various conductivepatterns on printed circuit boards 103 and 107. The conductive patternon printed circuit board 107, which is electrically connected to theground connection 105, also leads to a ground wire in the ribbon cable109 and is electrically connected thereto.

The high voltage connection to the bimorph is shown at 111. This highvoltage connection is either a low temperature eutectic solder, such asindium/bismuth, or a dab of conductive epoxy coupling the metalized area48 on the surface S3 of the bimorph to a conductive pattern formed onthe underside of the printed circuit board 103. This conductive patternleads to and is electrically connected to a high voltage wire in theribbon cable 109. The ribbon cable 109 can go to a remotely locatedbimorph driver circuit in the embodiment shown in FIG. 9.

A better understanding of the manner in which the electrical connectionsare made may be gained by referring to FIGS. 10 and 11. FIG. 10 shows aside view on a larger scale of the bimorph high voltage and groundconnections, while FIG. 11 shows a top view of the high voltage andground connections. The particular embodiment shown in FIGS. 10 and 11uses a single printed circuit board 113 as the substrate. A high voltageconductor 115 is formed on the surface of the printed circuit board as ametalized trace. A ground conductor 117 is also formed on the surface ofthe printed circuit board 113 as a metalized trace. In FIG. 11 threeindividual high voltage traces 115, 119, and 120 are shown so that theindividual bimorphs 121, 123, and 125 may be individually driven withsignals indicating the light intensity to be emitted from the pixelscontrolled by those particular bimorphs. A single shared ground trace117 is also shown. Each of the bimorphs in FIG. 11 is both mechanicallyaffixed to the substrate 113 and electrically connected to thecorresponding high voltage trace by a dab of conductive epoxy, shown asglue drops 127, 129, and 131 in FIG. 11. The metalized patterns on thesurfaces S1 of the three bimorphs in FIG. 11 are shown connected to theground trace 117 by individual glue drops of conductive epoxy 133, 135,137, and 139. The surfaces S4 on the underside of the bimorphs shown inFIG. 11 are coupled by conductive epoxy drops 141, 143, and 145 to theshared ground trace 117. These latter glue drops are shown in phantom inFIG. 11. The epoxy glue drops shown in FIG. 11 serve not only to makethe electrical connections but also serve as the mechanical mounting ofthe nonshutter end of the bimorph to the substrate 113.

Referring to FIG. 12, there is shown one possible embodiment for a shelflayout of sixteen adjacent bimorph electrical modulators. In thisparticular embodiment, the fiber optic input light guides 20 are bundledtogether at the left side of the board to form an input light bus 150.The individual input light guides then branch off to their individualbimorphs, as shown generally at 153. An edge connector 155 allows power,ground, and logic level signals to be input to the board to control thelight intensity modulation of each bimorph. A 16-channel high voltagedriver (not shown) is mounted on the board generally at 157. Theincoming pixel data for each bimorph is coupled to the 16-channel highvoltage driver by one of the conductive metallic traces etched on thesurface of the shelf, which is typically a printed circuit board orother insulating-type substrate material. Phenolic printed circuit boardis preferred, however, since techniques for forming conductive strips onsuch boards are well known. The high voltage signals from the highvoltage driver to the bimorph center electrodes are then conducted tothe bimorphs by metallic conductive traces on the printed circuit boardsubstrate 23. The plan view of FIG. 12 is shown without the top stop inposition so as to expose the details of the interconnections of thebimorphs to the edge connector and the high voltage driver. The metallictrace for the ground plane 117 is also not shown in FIG. 12. The highvoltage connection in FIG. 12 is shown as a solder pad, as used incertain embodiments. This solder pad is typically a copper plate whichis soldered to the metalized pattern for the high voltage electrodeusing the low temperature eutectic bismuth/indium solder. Prior to thisstep, a wire is bonded to this copper pad and to the particular highvoltage trace conductor on the surface of the printed circuit board 23,which corresponds to the light intensity signal for that particularbimorph. In alternative embodiments, the connective epoxy method offorming the high voltage and ground plane connections, as shown in FIG.11, may be used.

The preferred embodiment of the typical shelf layout uses the conductiveepoxy electrical connection structure of FIG. 11 and also uses thelamination sequence shown in FIG. 2. This allows the input light guide20 for each bimorph to pass through from the front edge 159 of the boardstraight back to the rear edge 161 of the board. This eliminates theneed to bend all the fibers to bring them together at one side of theshelf as the light bus 150. Other than these differences, the layout ofthe shelf is substantially the same as shown in FIG. 12.

An electrooptic modulator module may be constructed using a plurality ofthe shelves as shown in FIG. 12. Such a module is shown in FIG. 13. InFIG. 13 a plurality of shelves, as shown in FIG. 12, are stackedvertically in a framework with sixteen shelves stacked vertically, eachshelf containing sixteen bimorphs. The input light guides are coupled toa tricolor light bus 160, which contains three fiber optic light guides,each carrying feeder light of one of the three primary colors red,green, or blue. In color display applications, each pixel will becomprised of one red bimorph, one green bimorph, and one blue bimorph.The relative hue and intensity of each pixel then will be controlled bythree signals which modulate the relative intensity of each of the threeprimary colors. The overall brightness level of that pixel can becontrolled by modulating the light intensity of all three colorssimultaneously by equal amounts. This is done by changing the duty cycleof all three primary colors for that pixel simultaneously to increasethe on time or decrease the on time.

In the embodiment shown in FIG. 13, the tricolor light bus 160 iscoupled to the fiber optic bundles of light guides that run down theleft edges of the shelves constructed in accordance with the embodimentshown in FIG. 12. If shelves of the structure shown in FIG. 2 are used,the tricolor light bus 160 is coupled to the input light guides 20 asthey emerge from the back edges 161 of the shelves. In some embodiments,a fiber optic edge connector will couple the tricolor light bus to theback edges 161 so as to couple light into the input light guidesemerging from these back edges. The edge connectors 155 couple to aplurality of buses in a back plane surface 165. These power, ground, andlogic signals in the back plane 165 emerge from the electroopticmodulator controller 167. The purpose of this controller 167 is toconvert the brightness level and hue information for each pixel into asuitable analog or digital signal to control the application of highvoltage to the corresponding bimorph which is modulating light for thatpixel position in the display. The particular modulations scheme chosendepends upon the user's application. Therefore, the details of aparticular modulation-type controller will not be specified in greatdetail. However, one suggested controller architecture is shown in FIG.14.

Referring to FIG. 14, there is shown one possible architecture for acontroller for the type of bimorph light modulator disclosed herein. Inthis architecture, the desired light intensity for each pixel, or foreach color component of each pixel arrives on the line 169 as a digitalor analog signal. In the preferred embodiment, the magnitude of thissignal in terms of either its analog amplitude or its binary value willpresent the gray-scale value of desired intensity for that pixel. Thisgray-scale value is converted to a corresponding duty cycle by theconverter circuitry 171. Because pulse-width modulation has beenselected for this example of a controller architecture, the convertercircuitry 171 must convert the magnitude of the incoming signal on line169 to pulse widths having on time and off times which establish a dutycycle which will cause an average light flux to emerge from that pixelwhich will be perceived as the light intensity defined by the signal onthe line 169. There are many ways of converting the gray-scale value toa duty cycle, and those skilled in the art will be able to devisesuitable circuitry to accomplish this function.

The converter circuitry 171 outputs a duty cycle control signal on aline 173. This duty cycle control signal causes a high voltage drivercircuit 175 to gate high voltage onto a line 177 during the on time ofthe duty cycle control signal on the line 173, and causes the highvoltage driver 175 to block high voltage from being applied to the line177 during the off time of the duty cycle control signal on the line173. The line 177 represents the electrical connection to the centerelectrode of the corresponding bimorph. The bimorph then opens andcloses the light path in the gap 24 in accordance with the duty cycle ofthe high voltage on the line 177. The input light to the bimorph in theinput light guide 20 is represented by the light pathway 179 in FIG. 14.The pulses of output light in the output light guide 22 are representedby the line 181 in FIG. 14. Those skilled in the art will appreciatethat different forms of modulation other than pulse-width modulation mayalso be used to control the bimorph light modulator. Further, manydifferent architectures are possible for the controller structure whichimplements the selected modulation scheme. All such controllerstructures and modulation schemes which can control the average lightflux emerging from the output light guide 22 in accordance with an inputelectronic signal defining the desired pixel light intensity areintended to be included within the scope of the claims appended hereto.

Although the invention has been described in terms of the preferred andalternative embodiments disclosed herein, those skilled in the art willappreciate many changes which could be made to these structures andmethods without departing from the true spirit and scope of theinvention. All such changes and modifications are intended to beincluded within the scope of the claims appended hereto.

What is claimed is:
 1. A light modulator comprising:a substrate; aninput fiber optic light guide mounted on said substrate and having anoutput end; an output fiber optic light guide having an input endadjacent to but separated from the output end of said input guide by agap and situated so as to capture at least some of any light emergingfrom the output end of said input light guide; a bimorph having ashutter attached to an end thereof and having the opposite end mountedto said substrate such that said shutter cuts off light coupling betweensaid input light guide and said output light guide when said bimorph isin a first predetermined state and such that said shutter does notprevent said coupling of light when said bimorph is in a secondpredetermined state; and driver means for applying electric fields tosaid bimorph of the proper polarity and duty cycle to cause said bimorphto cycle between said first and second predetermined states with a dutycycle controlled by the magnitude of a desired light intensity signal.2. The apparatus of claim 1 wherein said bimorph is laminated withultraviolet setting glue.
 3. The apparatus of claim 1 further comprisinga top stop and a bottom stop for limiting the range of movement of saidbimorph and to damp resonant vibrations of said bimorph when in closeproximity with said top stop or said bottom stop.
 4. The apparatus ofclaim 3 wherein said bimorph is wide enough to cause viscous damping bythe surrounding air to tend to suppress bouncing by said bimorph offeither said top stop or said bottom stop.
 5. The apparatus of claim 1wherein said driver means couples a first potential to a first metalizedpattern in the middle of said bimorph and couples a second potential tosecond and third metalized patterns on opposite sides or the patterncoupled to said first potential.
 6. The apparatus of claim 5 whereinsaid first metalized pattern is etched back from the edges of saidbimorph to lessen the likelihood of electrical short circuits betweensaid first and second potentials and to eliminate electrostatic pinning.7. The apparatus of claim 6 wherein the bimorph is comprised of a firstpiezoelectric film which is shorter than a second piezoelectric film towhich is bonded and wherein said first metalized film is on the side ofsaid second piezoelectric film laminated to said first piezoelectricfilm and wherein said second metalized pattern is on the opposite sideof said second piezoelectric film to said first metalized pattern andhas an unmetalized area directly under the location of the electricalcontact to said first metalized pattern.
 8. The apparatus of claim 4further comprising fulcrum means for binding said bimorph at a locationbetween its two ends to said substrate and for forcing said bimorph toassume a shallow angle with said substrate sufficient to preventhydrostatic sticking of said bimorph to said substrate when said bimorphis resting on said bottom stop.
 9. A light modulator comprising:asubstrate support structure; a first piezoelectric film strip; a secondpiezoelectric film strip; a ultraviolet setting glue binding said firstpiezoelectric film strip to said second piezoelectric film strip; afirst electrode interposed between said first and second piezoelectricfilm strips for coupling to a first potential; a second electrodecovering a surface of said first piezoelectric film strip on theopposite side of said film from said first electrode for coupling to asecond potential; a third electrode covering a surface of said secondpiezoelectric firm strip on the opposite side of said film as said firstelectrode; a shutter attached to the end of the laminated structurecomprised of said first and second film strips; an input light guideaffixed to said substrate and having an output end; an output lightguide affixed to said substrate so as to be having an input end alignedwith the light path of any light emerging from said output end of saidinput light guide and separated from the output end of said input lightguide; means for attaching said laminated structure to said substratesuch that said shutter may be moved into and out of the gap between theoutput end of said input light guide and the input end of said outputlight guide in accordance with the potentials applied to saidelectrodes.
 10. The apparatus of claim 9 further comprising a top stopaffixed to said substrate so as to limit the magnitude of movement ofsaid laminated structure in the direction of removing said shutter fromthe light path between said input and output light guides to a movementadequate to completely remove said shutter from said light path in saidgap but not as large as the complete magnitude of tip movement that thelaminated structure would cause when a predetermined potential wasapplied to said electrodes.
 11. The apparatus of claim 9 wherein saidfirst electrode is a first metalized film pattern whichcovers all butthe edge portions of the surface of said first piezoelectric film facingsaid second piezoelectric film.
 12. The apparatus of claim 11 whereinsaid second piezoelectric film strip is shorter than said firstpiezoelectric film strip such that a portion of said first metalizedfilm pattern is exposed and further comprises an electrical conductorelectrically coupled to said first metalized pattern for coupling sameto said first potential and wherein said second electrode is a metalizedfilm covering all of the underside of said first film strip except thearea under the electrical connection between said electrical conductorand said first electrode.
 13. The apparatus of claim 12 wherein saidthird electrode is a metalized film which covers substantially all thesurface of said second piezoelectric film strip on the opposite side ofsaid second film strip from said first metalized film except a portionof the surface closest to said electrical connection to said firstmetalized film.
 14. The apparatus of claim 9 further comprising a bottomstop which limits the movement of said laminated structure in thedirection to insert said shutter into the light path in said gap. 15.The apparatus of claim 14 wherein said means for attaching includesmeans for supporting said laminated structure at the end opposite theshutter end and at a position intermediate between said shutter end andsaid end opposite said shutter.
 16. The apparatus of claim 18 whereinsaid means for supporting at said intermediate position causes saidlaminated structure to rest at an angle to said substrate when saidlaminated structure is at rest on said bottom stop.
 17. A lightmodulator comprising:a substrate; an input light guide affixed to saidsubstrate and having an output end; an output light guide affixed tosaid substrate so as to be coincident with the light path emerging fromsaid input light guide and having an input end spaced from said outputend; a bimorph affixed to said substrate at one end and having a freeend and registered in its affixation to said substrate so that its freeend is registered in a predetermined position adjacent to the gapbetween said input end and said output end; a shutter affixed to the endof said bimorph so that said shutter is moved into and out of the lightpath between said output end and said input end in accordance with thevoltage applied to said bimorph.
 18. The apparatus of claim 17 furthercomprising support means coupled to said bimorph to cause said bimorphto rest at an angle to said substrate when said bimorph shutter isoccluding the light path.
 19. The apparatus of claim 18 furthercomprising means for limiting the rang of movement of said shutter andsaid bimorph to a range sufficient to allow the bimorph to remove theshutter completely from said light path but not the complete range ofmotion of said bimorph if its movement were not limited.
 20. Theapparatus of claim 19 wherein said bimorph includes at least threeelectrodes, one of which is for connecting to a first potential sourceand two of which are for connecting to a second potential source andwherein said electrodes are metalized films on predetermined surfaces ofsaid bimorph and wherein said metalized films are patterned on thevarious surfaces so as to minimize the possibilities of shorts betweensaid first and second potential sources and so as to minimizeelectrostrictive behavior of said bimorph in the vicinity of theelectrical connections to said metalized films.